1. Field
Embodiments relate to a nonvolatile memory device, a storage system having the same, and a method of driving the nonvolatile memory device, and more particularly, to a nonvolatile memory device having improved reliability of a read operation, a storage system including the nonvolatile memory device, and a method of driving the nonvolatile memory device.
2. Description of the Related Art
A phase Change Random Access Memory (PRAM) stores data using a phase change material, for example, a chalcogenide alloy. A state of the phase change material changes to a crystalline state or an amorphous state when the phase change material is heated or cooled. For example, when the phase change material is in a crystalline state, it may have a low resistance, and the crystalline state is defined as “set” or data “0.” When the phase change material is in an amorphous state, it may have a high resistance, and the amorphous state is defined as “reset” or data “1.”
A read operation of a PRAM may be described as follows. First, when a PRAM is to read, current may be applied to a selected cell, and pass through current that depends on the resistance of the PRAM is generated. A sense amplifier compares a level of a sensing node, which may change depending on the pass through current, with a reference level, and identifies data as “0” or “1.”
However, a resistance of the PRAM may decrease as temperature rises. For example, a set resistance of 6 kΩ at 25° C. may change to 3.45 kΩ at 85° C., and a reset resistance of 150 kΩ at 25° C. may change to 50 kΩ at 85° C. The margin between the set resistance and the reset resistance may decrease because the decreasing extent of the reset resistance is higher than that of the set resistance; therefore, the sense amplifier may not correctly discriminate between the set state and the reset state. Further, in the case where the reset resistance greatly decreases, an operation error may occur in which the sense amplifier erroneously senses the reset state as the set state.